There are three British species of scorpionflies (genus Panorpa, order Mecoptera). They're not easy to tell apart, as the wing markings can vary and are not regarded as a reliable distinction. However, a good close-up photo of the males can often be identified, as long as it clearly shows their 'scorpion sting' (not actually a sting at all, just the genital capsule). Unfortunately, females can't be identified from photos.

In each case it is the shape of the paired 'callipers' (also called 'hypovalves') that you need to look at, on the surface of the male's genital capsule. These are well illustrated in an excellent blog post by Tim Worfolk:http://thetopshamnaturalist.blogspot.com/2010/05/scorpion-flies.html [Edit at June 2015 - this link is no longer working, sadly, it was a really helpful guide!]